The present invention relates to a method and apparatus for regulating a two-dimensional vector of a controlled system with a discrete-value final controlling element having a limited switching frequency.
Discrete-value final controlling elements only have a limited number of different states which can be set at their outputs. If an intermediate value is to be generated, an approximation is achieved by switching back and forth between adjacent states. The approximate value to be generated is a value averaged over time. The low-pass effect which is generally present in the regulation segment in technical systems smoothes the alternating variable produced.
If a two-dimensional vector of a controlled system is to be set, it is insufficient to consider each dimension separately, since they are in part dependent upon one another. Therefore, a change in the state of the final controlling element causes a change at more than one output.
Known control methods generally proceed from a one-dimensional approach. However, such an approach fails to take into consideration the manner in which the different outputs of the final controlling element depend upon each other as well as the internal states of the final controlling element.
Direct current regulation for rotary current drives using pulse inverters is described in "Regelungstechnische Praxis," Volume 24, 1983, Issue 11, pages 472 to 477. A very simple solution in terms of equipment is represented by two-point regulation. Such a regulation consists of three two-point regulators subjected to hysteresis. These regulators always cause a status change if the difference between the reference value and the actual value exceeds a certain limit. The system can therefore follow a changing reference value with maximum speed. This regulator type is used in intermediate circuit voltage inverters for regulating phase currents. Since the resulting phase currents are dependent upon one another, the switching states are not always optimal.
If the reference value periodically passes through a certain function, then suitable switching time points (with reference to the period) can be calculated in advance. The dependence of several phases on one another can be possibly taken into consideration. The disadvantage of this method is that a separate pulse pattern has to be calculated for each operating point (period duration and level). Each shift of the operating point requires a transition between pulse patterns. This transition can take a long time, in some cases, since a change in pulse pattern is only permitted at certain times within a period. At the same time, errors occur during the dynamic process, since the pulse pattern is not operated at its optimum point.
A one-dimensional process (two-point regulator with hysteresis) can be expanded to a multi-dimensional case. For this process, a hysteresis region around a reference value is predetermined instead of the hysteresis width. As soon as the actual value seeks to leave the region, a switch to another state takes place, which brings the actual value back to the inside part of this hysteresis region. With this method, the actual value is always kept within the vicinity of the reference value. Even if the reference value changes, quick following is guaranteed. The switching frequency can be influenced by the size of the hysteresis region, but cannot be precisely predetermined.
Such a multi-dimensional method is described as predictive current regulation in "Messen-Steuern-Regeln," No. 13, June 1989, pages 20 to 23, and "EPE," 1987, pages 647 to 652, entitled "New Predictive Control Strategy for PWM-Inverters." A further example of predictive current regulation with optimization in reach time for a pulse inverter is described in "IPEC," Tokyo 1983, pages 1665 to 1675. A multi-dimensional method for a GTO-I inverter is described in the lecture "Four Quadrant AC-Motor Drive with A GTO Current Source Inverter With Low Harmonics And On Line Optimized Pulse Pattern" by O. Hintze and D. Schroder, printed in the "IPEC" Conference Minutes, Tokyo 1990, pages 405 to 412.
With this method, the reference value can be reached by approximation, but it is not set as a mean value over time. It is possible that the trajectory of the actual value stays in a part of the hysteresis region for an extended period of time. This results in a mean value which is not equal to the reference value in every direction. This error has to be compensated with another regulator. This effect is particularly disruptive with large hysteresis regions and, therefore, small switching frequencies.
With a multi-dimensional method, a relatively low switching frequency and good control dynamics are achieved. The disadvantages of the method with precalculated pulse patterns are avoided in this method. However, even in this method the switching frequency cannot be predetermined precisely because it depends on the operating point at each point in time and varies greatly. Another disadvantage of this method is that the voltage is regulated, for example, but not the integral above the voltage, i.e. the current. For example, it is possible that the mean value of the voltage only becomes zero in one direction, and thus the current in the orthogonal direction to the voltage deviates significantly from the reference value for an extended period of time. Simply by activating the inverter, an error in the current is caused, which has to be balanced out again with a regulator which acts on the hysteresis region.
There is a need for a method and a switching arrangement for regulating a two-dimensional vector of a controlled system with a discrete-value final controlling element having a limited switching frequency that avoids the aforementioned disadvantages.